The present invention relates to an improved transmission electron microscope.
In a transmission electron microscope (TEM), an aperture plate having an objective aperture is disposed behind the objective lenses. This plate selects either bright field imaging or dark field imaging.
FIG. 1 is a schematic diagram showing the electron beam path in a conventional electron microscope. Optical analogs of the electron lenses are schematically shown in the FIGURE. A specimen 1 to be examined is placed in the magnetic field which is produced by a front objective lens 2 and a rear objective lens 3. The lenses 2 and 3 are disposed, respectively, above and below the specimen 1. The electron beam 4 that is to be directed to the specimen 1 is emitted by an electron gun 5 and diverges. The beam 4 is focused by first, second, and third focusing lenses 6a, 6b, 7, respectively, and the front objective lens 2 in turn. The electron beam transmitted through the specimen 1 passes through an objective aperture formed in a plate 8 and is directed into an imaging lens system 10 containing intermediate lenses.
In FIG. 1, the electron beam 4 is caused by the focusing lens 7 to produce a crossover image A of the beam at a position between the focusing lens 7 and the objective lens 2. Electrons diverging from the crossover image (point A fall on the specimen 1 through the objective lens 2. The electron beam transmitted through the specimen 1 is focused at the position of the aperture plate 8 by the rear objective lens 3, thus creating a crossover image B. The aperture plate 8 is disposed in the back focal plane defined by the rear objective lens 3. After passing through the aperture plate 8, the beam is directed into the imaging lens system 10 which is so set up that the intermediate lenses and other elements together create an electron micrograph on a fluorescent screen 11.
A crossover image (diffraction pattern) B as shown in FIG. 2 is formed on the aperture plate 8 having the objective aperture. This diffraction pattern varies, depending on the kind of specimen, but in principle consists of a central spot S.sub.0 (a so-called zero order spot) and peripheral spots S.sub.200, S.sub.220, etc. The central spot is formed from the electron beam which passed through the specimen without being scattered or diffracted. The peripheral spots are created from the electron beams diffracted toward certain directions by the specimen. When the objective aperture 8a in the aperture plate 8 is aligned with the zero order spot S.sub.0 as indicated by the solid line in FIG. 2, then only the electron beam which is neither scattered nor diffracted by the specimen passes through the aperture 8a, creating a bright field image on the fluorescent screen 11. When the aperture 8a is aligned with a desired one of the peripheral spots, such as the spot S.sub.200 as indicated by the broken line in FIG. 2, only the electron beam diffracted toward a certain direction passes through the aperture 8a. As a result, a dark field image is created on the fluorescent screen 11 under desired diffraction conditions.
In this way, the aperture plate 8 is used to pass only the beam contributing to the formation of a desired spot. Therefore, the aperture plate 8 is required to be disposed correctly on the plane where the crossover image B is formed. Preferably, the diameter of the aperture 8a is so set that the aperture does not reach to any other spot. That is, the desired diameter is approximately between 10 .mu.m and 20 .mu.m.
The magnification of a transmission electron microscope can be set at will by changing the intensity of the imaging lens system. In this case, the area of the region of the specimen irradiated with the electron beam is varied according to the magnification. Specifically, the area is narrowed at higher magnifications and enlarged at lower magnifications. The irradiated area is set by the operator by changing the magnitude of the excitation current supplied to the final-stage focusing lens (in the case of the electron microscope shown in FIG. 1, the third focusing lens 7). Since the brightness of the image can be varied by changing the area of the irradiated region, this final-stage focusing lens is normally called the brightness lens.
When the area of the irradiated region is changed in this way by adjusting the intensity of the brightness lens 7, the position at which the crossover image is formed is shifted along the z-axis, or the optical axis of the electron beam, by the objective lenses 2 and 3 having constant intensities. The manner of the shift is illustrated in FIGS. 3(a)-3(c), where the magnification is set to a high value, an intermediate value, a low value, respectively. In FIGS. 3(a)-3(c), the broken lines show diffracted electron beams.
Where the position at which the crossover image B is formed coincides with the aperture plate 8 having the objective aperture as shown in FIG. 3(b), only the electron beam forming a desired spot contributes to the formation of an image and so the image is correctly obtained under the desired diffraction conditions. However, where the position at which the crossover image B is formed does not coincide with the aperture plate 8 at high and low magnifications (FIGS. 3(a) and 3(c)), electron beams forming undesired spots pass through the aperture plate 8a and contribute to the formation of the image, as well as the electron beam forming the desired spot. Consequently, the image is not obtained exactly under the desired conditions. In the case shown in FIG. 3(c), the aperture plate 8 excessively limits the field of view.
One conceivable countermeasure is to make the aperture plate 8 movable along the z-axis so that an adjustment is made according to the magnification to maintain the plate 8 at the position at which the crossover image B is formed. However, this scheme cannot be put into practical use because it is difficult to secure space sufficient to install an aperture plate shift mechanism within the present-day electron microscope.
With respect to the unwanted limitation on the field of view, it is considered that the diameter of the aperture 8a is set to a large value of about 50 to 100 .mu.m. If such a large-sized aperture is used, then plural diffraction spots are situated within the aperture. This makes it impossible to strictly select desired diffraction conditions. Also, the resolution may deteriorate.
These problems also take place where only the intensity of the third focusing lens 7 is adjusted without changing the magnification, for varying the area of the irradiated region.
An electron microscope equipped with an auxiliary lens between a set of objective lenses and a brightness lens is disclosed in U.S. Pat. No. 4,633,085. This auxiliary lens is used to enable the microscope to be switched between the TEM mode and the analysis mode while keeping the correspondence between the specimen positions that are irradiated in the TEM mode and the analysis mode, respectively. Therefore, the known auxiliary lens differs in purpose and form from the novel auxiliary lens.